There are many measurement problems which ultimately can be reduced to a measurement of the time period between two events. Measurement of such parameters as linear or rotational velocity, linear or rotational acceleration, temperature, pressure, frequency, energy, power, current, voltage or position can be made using an accurate detection of a time period. Such time periods are often measured via a stable clock and counter. The accuracy and resolution of such time measurements thus depends upon the speed of the stable clock and the resolution of the counter. In such circuits the frequency of the clock generator is generally the limiting factor. Increased resolution of measurement generally requires clock generator circuits which operate at higher speeds and are more expensive, and higher speed counter circuits, at least for the highest speed stages, which are likewise more expensive.
In many applications the use of conventional clock and counter circuits yields less resolution than is desirable. It is known in the prior art to employ various cascading techniques to improve the resolution of such time measurements. This requires repeated measurements of the time period in question. The time periods are cascaded so that a number of such time periods are added together and the sum time measured using a clock and counter system. This technique is often called recirculation. Summing additional examples of the time period enables increased resolution by applying the time resolution capacity of the system over plural time periods. An increase in the time required to make the measurement is a natural consequence of this recirculation technique. This increase in time of measurement is a disadvantage, particularly when the measurement is employed in a feedback and control system. It is well known that delays in feedback and control systems contribute to instabilities. Thus the goals of increased resolution and decreased time of measurement are antagonistic.
A study of the nature of feedback and control loop design indicates that great resolution and fast speed are not generally required simultaneously. Great measurement speed is required during times when the measured quantity is changing moderately or rapidly. During times when the measured quantitY is changing most feedback and control systems exhibit significant errors in measurement. Great resolution is not required during such times because this resolution would be outweighed by inherent errors in the measurement process. However, during periods of motion speed of measurement is essential to reduce control instabilities inherent in delays. Conversely, great resolution is typically required only when the measured quantity is unchanging or nearly unchanging. During such times greater delays in the measurement process can be tolerated without introducing control instability. Under unchanging conditions the need for speed is reduced while the need for resolution is increased.
Thus while great resolution and great speed cannot easily be achieved simultaneously, they are not needed simultaneously. During times when great speed is required, great resolution is typically not helpful. During times when great resolution is needed, slower measurements may be tolerated. Thus a measurement system which can provide great speed during times when the measured quantity is changing and great resolution when the measured quantity is unchanging or nearly unchanging would often be as useful as a system which provides great speed and great resolution simultaneously.